The density measurement of high pressure wet steam in small pipes is needed in many industrial processes. One example is in crude oil extraction where high pressure, wet steam injection is sometimes used for cracking underground formations of heavy crude oil. In this application, it is desirable to monitor the density of wet steam so that the corresponding quality of steam is maintained within desired limits. These limits on the density of wet steam are usually set such that the quality of steam is high enough to crack and soften the crude formation and yet wet enough to remove depository particles from the boiler. Another example is found in the electric power industry where steam produced from a steam generator is used to drive a turbine producing electricity. In this case, it is very important to measure the density in order to ensure that the density is below the allowed limit so that the turbine functions properly.
There have been, in the past, various proposed methods of measuring the density of a steam-water mixture and they use, (a) quick closing valves, (b) impedance probes, (c) optical probes, and (d) ultrasound probes. All these methods, however, possess one or more of the following shortcomings:
(a) the method is intrusive and disturbs the flow of steam, PA0 (b) the method disturbs the routine operation of processes, PA0 (c) the method is not applicable because of a thick steel pipe wall, PA0 (d) the method is not sensitive, and/or PA0 (e) the device is not portable.
Other methods of measuring the density of high pressure, wet steam use the attenuating/scattering characteristics of radiation which the wet steam possesses. The wet steam is defined as a steam-water mixture having a high void fraction. The void fraction is a volume fraction occupied by steam phase).
These methods operate on the attenuation/transmission of various radiations like X-rays, .beta. particles and .gamma.-rays. However, X-rays and particles cannot penetrate thick metal pipe walls. Gamma rays can penetrate thick pipe walls but is very insensitive in the high void region (wet steam). The scattering of epithermal/fast neutrons has proven very sensitive in the low and intermediate void fraction region and in pipes of bigger diameter (D&gt;50 mm). However, it becomes quite insensitive in the high void fraction region, particularly in small pipes because the probability of thermalization is small.
U.S. Pat. No. 4,243,886, Jan. 6, 1981, (Untermeyer), discloses a technique for determining the hydrogen content of such materials as wood or concrete, or the quantity of water in the human lung. The technique uses a source of fast neutrons and a thermal neutron detector. The source and the detector are positioned on one side of an object whose neutron moderating or absorbing properties are being measured. The fast neutrons from the source are moderated or absorbed to generate thermal neutrons in passing through the object and the detector detects thermal neutrons emerging from the object. A standard neutron absorber is used to derive the difference between the object and the standard in the thermal neutron counts at the detector. The difference is an indication of the moderating or absorbing properties of the object.
In U.S. Pat. No. 3,350,564, Oct. 31, 1967, (Bonilla et al), a method of measuring the void fraction of boiling water using neutron attenuation is described. This patent uses small amounts of boron which are dissolved in the water to absorb the neutrons and a beam of low energy neutrons is transmitted through the boiling water. The amount of attenuation is related to the void fraction of the water.